Effect of Concentration and Temperature on the Structure and Ion Transport in Diglyme-Based Sodium-Ion Electrolyte

Abstract

Glyme-based sodium electrolytes show excellent electrochemical properties and good chemical and thermal stability compared with existing carbonate-based battery electrolytes. In this investigation, we perform classical molecular dynamics (MD) simulations to examine the effect of concentration and temperature on ion−ion interactions and ion− solvent interactions via radial distribution functions (RDFs), mean residence time, ion cluster analysis, diffusion coefficients, and ionic conductivity in sodium hexafluorophosphate (NaPF 6 ) salt in diglyme mixtures. The results from MD simulations show the following trends with concentration and temperature: The Na + ---O(diglyme) interactions increase with concentration and decrease with temperature, while the Na + ---F(PF 6 − ) interactions increase with concentration and temperature. The mean residence time suggests that Na + ---O(diglyme) are significantly longer lived compared with that of Na + ---F(PF 6 − ) and H (diglyme)---F(PF 6 − ), which shows the affinity of diglyme to the Na + ions. The ion cluster analysis suggests that the Na + ions largely exist as solvated ions (coordinated to diglyme molecules), whereas some fractions exist as contact-ion pairs, and negligible fractions as aggregated ion pairs, with the latter two increasing slightly with temperature and more with ion concentration. The magnitude of the diffusion coefficients of Na + and PF 6 − ions decreases with concentration and increases with temperature, where the Na + ion has slightly lower mobility compared with the PF 6 − anion. The simulated total ionic conductivities show qualitative trends comparable to experimental data and highlight the need for the inclusion of ion−ion correlations in the Nernst−Einstein equation, especially at higher concentrations and lower temperatures.

An Ultrastable Low-Temperature Na Metal Battery Enabled by Synergy between Weakly Solvating Solvents

“…Classical molecular dynamics (MD) simulation was introduced to explain the unexpectedly high ionic conductivity at the lower temperature region (−30 to −60 °C) (Figure b,c). The PF 6 – anions mainly locate in the outer layer of the solvation sheath, illustrating that a low fraction of contact ion pairs (CIPs) or anion aggregates (AGGs) are formed in the electrolyte with NaPF 6 salt, which is consistent with the Raman spectra (Figure S5) and the previous results. − In the typical solvation structure, there are six THF molecules in the solvation sheath for the electrolyte 1 M NaPF 6 -THF (inset in Figure b). In contrast, there are four THF molecules and two MeTHF molecules in the solvation sheath for NaPF 6 -THF/MeTHF (inset in Figure c).…”

Monolithic Interphase Enables Fast Kinetics for High‐Performance Sodium‐Ion Batteries at Subzero Temperature

“…The RDFs exhibit a maximum at 2.35 Å for Na + −O interactions, indicating that the sodium ion is surrounded by DG molecules as the first solvated sheath. The calculated coordination number (CN) of Na−O in 0.5M‐D (2.18) is higher than that in 1M‐D (1.99), and it further increases to 2.26 at −30 °C, which supports the conclusion that more DG enters the solvated sheath [13] …”

“…The calculated coordination number (CN) of NaÀ O in 0.5M-D (2.18) is higher than that in 1M-D (1.99), and it further increases to 2.26 at À 30 °C, which supports the conclusion that more DG enters the solvated sheath. [13] To gain insight into the practicality of 0.5M-D in subzero environments, LMNM'T//Na half cells were assembled to estimate the electrochemical performance, and the common electrolyte 1M NaPF 6 in propylene carbonate with 5 vol. % fluoroethylene carbonate (denoted as PC) was chosen as a comparison.…”

Ion correlations in quaternary ionic liquids electrolytes

“…Based on their analysis, they found that a change in the solvent concentration can suppress the anion-cation contribution to conductivity and increase the Li + transference number [29]. Similarly, the effect of concentration and temperature on the ion transport in diglyme-based sodium ion electrolyte were reported by Ardhra et al [30]. They showed that the ionic correlations have to be considered to correctly predict the experimentally observed trends in ion conductivity.…”